CO2 Electroreduction to Formate: Advancing toward Scalable Technologies.

Design methodology for mass transfer-enhanced large-scale electrochemical reactor for CO[formula omitted] reduction

Facile fabrication of binary g-C3N4/NH2-MIL-125(Ti) MOF nanocomposite with Z-scheme heterojunction for efficient photocatalytic H2 production and CO2 reduction under visible light

Due to land use intensification and drainage many peatlands have lost their C sink function. Consequently, rewetting has become an important strategy to mitigate increased greenhouse gas emissions from degraded peatlands. Whereas CO2 emissions decrease under reducing conditions upon waterlogging, CH4 production rates increase. The exact effect of rewetting may depend on the initial degree of degradation of a peatland and resulting peat quality. Therefore, the aim of this study was to elucidate waterlogging effects on C mineralization rates of peat from two contrasting sites. Near-surface peat soils from a long-term drained area and a rewetted site with newly formed floating mat were incubated under aerobic and anaerobic conditions for 90 days. CO2 and CH4 production rates were measured with weekly intervals. At the beginning and at the end of the incubation, liquid phase samples were taken and analysed for (in)organic ions, element stoichiometry, UV absorbance spectra and, DOC concentrations. When CO2 and CH4 production had reached steady states, we measured C-, N- and P-related hydrolytic enzyme activities of the peat. We expected that hydrolytic enzyme activities decrease, resulting in lower CO2 production rates, under anaerobic conditions. Furthermore, it was hypothesized that C mineralization rates of the pristine floating mat would exceed those of the degraded drained peatland due to higher availability of more labile organic matter in the former site. As expected, rewetting, as simulated by anoxic incubations, slowed CO2 production rates and activities of beta-glucosidase as compared to the oxic controls. Moreover, the availability of oxygen stimulated near-surface peat decomposition supported by a strong decrease in DOC concentrations after aerobic incubation in the degraded peat. However, the average rate of CO2 production was six times higher in the degraded drained site compared to the restored floating mat (189.84 and 29.76 μmol CO2 g dw-1 d-1, respectively). CH4 production from the long-term drained site began after 75 days of anoxic incubation and was almost negligible compared to the restored site (0.06 v. 0.46 g dw-1 d-1 after 75 days of incubation, respectively). Due to the high CO2 production rates measured at the drained site, it is unlikely that high peat recalcitrance was the cause of low CH4 production. In contrast to CO2 production rates, there were no significant differences in beta-glucosidase activities between the two sites. Probably other substrates than cellulose were involved in peat decomposition from the degraded site compared to decomposition of the floating mat. Therefore, this may either imply that degraded peat has an adapted community of microbes releasing enzymes that are able to breakdown a wide spectrum of organic sources, including aromatics. Or, alternatively, the build-up of phenolics in the Sphagnum-rich restored site inhibits hydrolytic enzyme activity and consequently leads to lower CO2 production rates. Thus, under anoxic conditions, overall low activities of hydrolytic enzymes partly supported the enzymatic latch paradigm. We have shown that rewetting slows CO2 production rates and may not result in immediate CH4 production. Moreover, peat quality and enzyme activities appear an important control on peatland restoration that requires further investigation.

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

Zn/Cu electrocatalysts were synthesized by the electrodeposition method with various bath compositions and deposition times. X-ray diffraction results confirmed the presence of (101) and (002) lattice structures for all the deposited Zn nanoparticles. However, a bulky (hexagonal) structure with particle size in the range of 1–10 μm was obtained from a high-Zn-concentration bath, whereas a fern-like dendritic structure was produced using a low Zn concentration. A larger particle size of Zn dendrites could also be obtained when Cu2+ ions were added to the high-Zn-concentration bath. The catalysts were tested in the electrochemical reduction of CO2 (CO2RR) using an H-cell type reactor under ambient conditions. Despite the different sizes/shapes, the CO2RR products obtained on the nanostructured Zn catalysts depended largely on their morphologies. All the dendritic structures led to high CO production rates, while the bulky Zn structure produced formate as the major product, with limited amounts of gaseous CO and H2. The highest CO/H2 production rate ratio of 4.7 and a stable CO production rate of 3.55 μmol/min were obtained over the dendritic structure of the Zn/Cu–Na200 catalyst at −1.6 V vs. Ag/AgCl during 4 h CO2RR. The dissolution and re-deposition of Zn nanoparticles occurred but did not affect the activity and selectivity in the CO2RR of the electrodeposited Zn catalysts. The present results show the possibilities to enhance the activity and to control the selectivity of CO2RR products on nanostructured Zn catalysts.

Bubble Formation in the Electrolyte Triggers Voltage Instability in CO2 Electrolyzers.

Membrane electrode assemblies (MEAs) with a sulfonated polysulfone (sPSU) as the proton-conducting phase were fuel cell evaluated at varying temperatures in over-humidified conditions. The sPSU was prepared by a direct polycondensation involving a commercially available sulfonated naphthalene diol monomer. The gas diffusion electrodes (GDEs) and MEAs were successfully fabricated and a thorough morphological study was subsequently carried out on GDEs with varying sPSU contents and ink solvents. The scanning electron microscopy and porosimetry studies revealed highly porous GDE morphologies at sPSU contents below . Double-layer capacitance measurements showed an almost fully sPSU-wetted electronic phase when the sPSU content was . The MEAs were prepared by applying the GDEs directly onto sPSU membranes. MEAs with a total Pt loading of were successfully fuel cell operated at . The MEAs showed mass-transport limitations in the range of , most probably caused by abundant water due to the overhumidified measuring conditions. The low resistance of the MEAs indicated a well-integrated structure between the GDEs and the membrane.

The development of CO2 utilization technologies has seen rapid progress during the past few years. In this area of research, electrochemical CO2 reduction (eCO2R) has been identified as one of the promising pathways. However, this process is yet to reach industrially relevant rates of product formation. In the eCO2R, the gas diffusion electrode (GDE) is the key component, with its architecture playing an important role. This review presents the latest advancements and opportunities in GDE structural design and materials selection, with a deep dive into the structure–performance relationship and its complex interplay in eCO2R. Many recent research efforts have focused on improving catalysts, gas diffusion structures (gas diffusion layers (GDLs) and porous hollow fiber walls), electrolytes, and interfaces in order to optimize key performance metrics such as activity, selectivity, and stability, which are often intertwined and can complicate design efforts. The basic configuration has transitioned from conventional planar GDEs to self-supported hollow fiber GDEs (HFGDEs), along with emerging advanced forms of planar GDE, such as mesh, woven, carbon-free, and heteroarchitectural designs. These advancements have led to enhanced triple-phase boundary formation and improved mass transfer, resulting in high-performance GDEs capable of achieving ampere-level current densities (∼3 A cm−2), high faradaic efficiencies (FE) for target products, and extended operational stability (>100 h). Further, we discuss current bottlenecks and provide perspectives aimed at offering new insights and guiding research directions to advance the development of industrially applicable GDE-based eCO2R systems and facilitate their practical implementation.

Benefiting from their unique structural merits, three-dimensional (3D) large-pore COF materials demonstrate high surface areas and interconnected large channels, which makes these materials promising in practical applications. Unfortunately, functionalization strategies and application research are still absent in these structures. To this end, a series of functional 3D stp-topologized COFs are designed based on porphyrin or metalloporphyrin moieties, named JUC-640-M (M = Co, Ni, or H). Interestingly, JUC-640-H exhibits a record-breaking low crystal density (0.106 cm3 g-1) among all crystalline materials, along with the largest interconnected pore size (4.6 nm) in 3D COFs, high surface area (2204 m2 g-1), and abundant exposed porphyrin moieties (0.845 mmol g-1). Inspired by the unique structural characteristics and photoelectrical performance, JUC-640-Co is utilized for the photoreduction of CO2 to CO and demonstrates a high CO production rate (15.1 mmol g-1 h-1), selectivity (94.4%), and stability. It should be noted that the CO production rate of JUC-640-Co has exceeded those of all reported COF-based materials. This work not only produces a series of novel 3D COFs with large channels but also provides a new guidance for the functionalization and applications of COFs.

In this work, we have designed and synthesized nickel-laden dendritic plasmonic colloidosomes of Au (black gold-Ni). The photocatalytic CO2 hydrogenation activities of black gold-Ni increased dramatically to the extent that measurable photoactivity was only observed with the black gold-Ni catalyst, with a very high photocatalytic CO production rate (2464 ± 40 mmol gNi-1 h-1) and 95% selectivity. Notably, the reaction was carried out in a flow reactor at low temperature and atmospheric pressure without external heating. The catalyst was stable for at least 100 h. Ultrafast transient absorption spectroscopy studies indicated indirect hot-electron transfer from the black gold to Ni in less than 100 fs, corroborated by a reduction in Au-plasmon electron-phonon lifetime and a bleach signal associated with Ni d-band filling. Photocatalytic reaction rates on excited black gold-Ni showed a superlinear power law dependence on the light intensity, with a power law exponent of 5.6, while photocatalytic quantum efficiencies increased with an increase in light intensity and reaction temperature, which indicated the hot-electron-mediated mechanism. The kinetic isotope effect (KIE) in light (1.91) was higher than that in the dark (∼1), which further indicated the electron-driven plasmonic CO2 hydrogenation. Black gold-Ni catalyzed CO2 hydrogenation in the presence of an electron-accepting molecule, methyl-p-benzoquinone, reduced the CO production rate, asserting the hot-electron-mediated mechanism. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO2 hydrogenation took place by a direct dissociation path via linearly bonded Ni-CO intermediates. The outstanding catalytic performance of black gold-Ni may provide a way to develop plasmonic catalysts for CO2 reduction and other catalytic processes using black gold.

Effect of gas diffusion layer and membrane properties in an annular proton exchange membrane fuel cell

The performance of electrocatalysts for the electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) is largely dependent on the ability to efficiently deliver CO2 to the active sites. A variety of reactor configurations have been explored in the literature that can be broadly classified as based on either liquid- or gas-phase reactant delivery. These configurations utilize a range of electrode types including metal plates, meshes, packed granules, and gas diffusion electrodes (GDEs).[1] Amongst these methods, the use of gas-phase reactor designs employing GDEs enables a dramatic increase in current density (typically an order of magnitude or larger) over liquid-phase reactor designs, where the low solubility and aqueous diffusivity of CO2 result in severe mass transport limitations. However, per existing literature, the performance of GDEs in various CO2 electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. Prior reports have demonstrated electroreduction of CO2 to formate (FA) on commercial tin nanoparticle (150 nm) loaded gas diffusion layers (GDLs) at current densities of 200 mA/cm2, 80% selectivity to FA, and cathodic potentials of -0.8 V vs. RHE.[2, 3] However, these and other studies have also reported that at higher catalyst loadings (thicker catalyst layers), which are desirable for high production rates, conversion efficiencies drop and undesirable side product formation increases due to reactant starvation. Reducing particle size typically enhances both catalyst utilization and activity per unit mass. This, in turn, may enable thinner catalyst layers. While synthesis methods exist for generating smaller (< 10 nm) particles, these particles must still be deposited on a GDL such that ionic and electronic contact can be maintained with the electrolyte and GDL, respectively. These critical interfaces are key to maximizing electrode performance in terms of product generation rate, selectivity, and catalyst utilization. Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” (EC) approach that used pulse and pulse-reverse electrodeposition to obtain highly dispersed and uniform Pt catalyst nanoparticles (~5 nm).[4-6] Moreover, since the catalyst was electroplated through an ionomer layer onto the bare GDL, the formed nanoparticles were inherently in both electronic and ionic contact within the GDE and, consequently, utilization was enhanced. Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a loading of 0.5 mg/cm2.[6] Here we investigate the electrodeposition of tin (Sn) onto commercially available GDLs through an EC process and benchmark our results against a state-of-the-art Sn nanoparticle catalysts (150 nm) spray-coated on a GDL. Electrolysis experiments are conducted in a three compartment W-Cell setup using the Sn-coated GDEs as cathodes and Pt/H2 counter electrode. We demonstrate that the EC GDE samples can exhibit up to 388 mA/cm2 total current density and 76% selectivity to formate at cathodic potentials of -0.8 V vs. RHE, representing a two-fold improvement in current density over both our benchmark electrode and existing reports using Sn-loaded GDEs prepared by conventional methods.[2, 3] We hypothesize that this enhancement arises from improved catalyst utilization, leading to high electrode activity. Surprisingly, SEM imaging of the EC GDE reveals Sn particles no smaller than the micron scale (~10 μm). Thus, we anticipate further improvement in electrode activity may be realized through suitable tuning of the EC waveform to yield nanoscale Sn particles (< 10 nm). In summary, the EC approach appears promising for fabricating active catalytic layers directly onto GDL substrates.

As electrochemical technologies such as batteries, fuel cells, and water electrolyzers advance and transform the electric and transportation sectors, there is increasing interest around the role of electrochemistry in sustainable chemical manufacturing. As an example, blending electrochemically generated carbon monoxide (CO) and hydrogen, derived from carbon dioxide (CO2) and water electrolyses respectively, could constitute a renewable scheme to produce syngas for Fischer-Tropsch gas-to-liquids processes1. Decades of fundamental research into electrochemical CO2 reduction (CO2R) coupled with emerging engineering and economic incentives have shifted the field’s focus towards high-performance, gas-fed electrolyzers. Catalyst-coated gas diffusion electrodes facilitate such reactor configurations by providing physical separation between the gaseous reactants and the liquid electrolyte. While high geometric-area-specific electrochemical activity has been demonstrated with commercial gas diffusion electrode materials for a variety of both CO- and hydrocarbon-selective metal catalysts2,3, longevity remains a challenge and performance decay is often attributed to electrode deficiencies. To date, most gas diffusion layers reported in the CO2R literature have been repurposed from fuel cell applications, in which the transport of water to and from the catalyst layer is crucial to device operation. To this end, in fuel cells, densely-packed microporous layers serve both as catalyst-layer substrates and effective media for water management. However, the efficacy of this layer as a barrier to liquid electrolyte flooding in CO2R is limited by increasing hydrophilicity upon exposure to reducing potentials and high local pH. New operando diagnostic techniques are needed to probe the stability of the gas-liquid interface in gas-fed CO2 electrolyzers with flowing liquid electrolytes4. In this presentation, we propose a new experimental approach for determining the relationship between cell operating conditions and the eventual degradation of CO2-to-CO faradaic efficiency. Specifically, we propose combining periodic in-situ electrochemical-double-layer-capacitance-based electrolyte wetting predictors with in-line gas chromatography characterization of CO2R products. Voltammetric- or impedance-based methods are often used to estimate electrochemically active surface area of porous carbons in supercapacitor applications, but have yet to be used to probe electrolyte wetting in CO2R gas diffusion electrodes5. To demonstrate this technique, we evaluate the flooding tolerance of silver-coated gas diffusion electrodes under a range of operating conditions and using a number of commercial gas diffusion layers. Additionally, we discuss the impact of electrolyte properties (e.g., composition, pH), electrode properties (e.g., PTFE-content, macroporous layer, cracking), and operating conditions (e.g., pressure differential, current density, temperature) on flooding phenomena to identify key descriptors that can inform the design of more resilient electrode configurations. Funding Acknowledgement We gratefully acknowledge funding support from the US Department of Energy SBIR Program Grant # DE-SC0015173 and Royal Dutch Shell plc through the MIT Energy Initiative. References (1) Bushuyev, O. S.; De Luna, P.; Dinh, C. T.; Tao, L.; Saur, G.; van de Lagemaat, J.; Kelley, S. O.; Sargent, E. H. What Should We Make with CO2 and How Can We Make It? Joule 2018. https://doi.org/10.1016/j.joule.2017.09.003. (2) Ma, S.; Sadakiyo, M.; Luo, R.; Heima, M.; Yamauchi, M.; Kenis, P. J. A. One-Step Electrosynthesis of Ethylene and Ethanol from CO2 in an Alkaline Electrolyzer. J. Power Sources 2016, 301, 219–228. https://doi.org/10.1016/j.jpowsour.2015.09.124. (3) Verma, S.; Lu, X.; Ma, S.; Masel, R. I.; Kenis, P. J. A. The Effect of Electrolyte Composition on the Electroreduction of CO2 to CO on Ag Based Gas Diffusion Electrodes. Phys. Chem. Chem. Phys. 2016, 18 (10), 7075–7084. https://doi.org/10.1039/C5CP05665A. (4) Higgins, D. C.; Hahn, C.; Xiang, C.; Jaramillo, T. F.; Weber, A. Z. Gas-Diffusion Electrodes for Carbon-Dioxide Reduction: A New Paradigm. ACS Energy Lett. 2018. https://doi.org/10.1021/acsenergylett.8b02035. (5) Verbrugge, M. W.; Liu, P. Analytic Solutions and Experimental Data for Cyclic Voltammetry and Constant-Power Operation of Capacitors Consistent with HEV Applications. J. Electrochem. Soc. 2006, 153 (6), A1237–A1245. https://doi.org/10.1149/1.2194610.

CO2 production has been followed by manometry in synchronous and asynchronous cultures of Schizosaccharomyces pombe prepared by elutriation from the same initial culture. The rate of production follows a linear pattern in synchronous cultures with a rate change once per cycle at the time of cell division. This pattern is most clearly shown in oscillations of the difference between values of the second differential (acceleration) for the synchronous and asynchronous cultures. The association between the rate change and the time of division is maintained during growth speeded up in rich medium and slowed down in poor medium and at lower temperature. It is also maintained after a shift-up in temperature. Results with wee mutants suggest that the association is with the S period rather than division itself. The rate and acceleration of CO2 production are approximately proportional to cell size (protein content) in asynchronous cultures. When synchronous cultures of the temperature-sensitive mutants cdc2.33 and cdc2.33 wee1.6 are shifted up to the restrictive temperature, the DNA-division cycle is blocked. The oscillatory pattern of CO2 production, however, continues for one to two cycles until the acceleration reaches a constant value, after which the oscillations are undetectable. This point is reached later in the double mutant and there is a phase difference in the oscillations compared to those in the single mutant. With both blocked mutants the 'free-running' oscillations are about 15% shorter than the normal cycle time. There are well-known examples of such oscillations in eggs but they are rare in growing systems.

The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni3N) nanosheets as an alternative to these metals. The Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g−1 h−1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO2 hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni3N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.